Did I read that right? Our MMM cars are NEGATIVE earth? This is important
for me to know as I am in the midst of doing the wiring on my L-1 Salonette
and don't really care to be frying things!
- Mike Hughes -t?t-
----- Original Message -----
From: Pat Mullen <pmullen@telus.net>
To: <hiro@octagongarage.com>
Cc: <mg-mmm@autox.team.net>
Sent: Wednesday, October 30, 2002 4:02 PM
Subject: Re: 3 brush dynamo
> Hi Hiro,
>
> How good to hear of a fellow MMM enthusiast who REALLY drives his car!
> (I used to own a 1933 J2 and then a 1934 J2 in 1952-54 when I was a
> student in England - I am now a Canadian with a 1936 NB nearing the end
> of a complete restoration).
>
> For some 30 years, between about 1936 to 1965 prior to the advent of
> alternators, and after the MMM cars had finished production, British
> cars controlled the VOLTAGE output from their dynamos with a
> "Compensated Voltage Control" (CVC) device. This measured the dynamo
> output voltage; the CVC control mechanism was placed in series with the
> dynamo field coil, between the dynamo + and - terminals. There was no
> "third brush". It worked quite well.
>
> Prior to this, in the days of our MMM cars in the early 1930s and
> before, the CURRENT was controlled by the third brush method in such a
> way that changes in engine speed made comparatively little change to the
> current supplied from the dynamo.
> The only ways to control the current on an MMM car were by
> (a) moving the third brush while doing maintenance on the car
> (b) switching resistance and or out of the field coil circuit, usually
> using the lighting switch.
>
> Here is an explanation of how method (a) worked on a negative ground
> ("earth") car like our MMMs:
> 1. On ANY dynamo, third brush or CVC, there is a magnetic field whose
> lines of force pass from one field coil to another, because one field
> coil is wound to produce a North pole on its inside face and the other
> field coil is wound to produce a South pole on ITS inside face. Such a
> dynamo at rest (but with the field coils' slight magnetism remaining
> from the last time they were energised) has straight lines of magnetic
> force.
> (The same principle applies to dynamos with four field coils - like my
> NB's dynamo. I'm unsure if your J2 has two or four field coils)
> 2. As the dynamo starts to revolve, it generates voltage; as this
> voltage increases, current flows in the armature wires; this in turn
> creates another, revolving, magnetic field which distorts the field
> produced by the field coils.
> 3. This distortion does not seriously affect the output of a dynamo
> designed to work with CVC, because the CVC is in control.
> 4. This type of distortion can be made to control the dynamo CURRENT
> output if it can be made to REDUCE THE STRENGTH OF THE EFFECTIVE
> MAGNETIC FIELD in the same proportion as the SPEED OF ARMATURE ROTATION
> INCREASES.
> 5. This can be accomplished by connecting one end of the field coil
> circuit to the positive output brush (as with the CVC dynamo) and the
> other end to a third, movable brush connected near the negative output
> brush in such a manner that any one segment of the armature passes the
> third brush just before it passes the negative output brush.
> 6. As you might expect, the nearer together the two brushes are, the
> higher the current output of the dynamo.
> 7. Note that the third brush method of control is much better than NO
> control but it is inferior to CVC.
> This is because whether the (battery plus lights etc) needs more or
> less
> current to maintain the correct 13.5 volts, the dynamo doesn't know or
> care.
> If the dynamo is set to produce about 8 amps at engine speeds above
> about 1500rpm, and your ignition coil takes 2 of these amps, AND you
> have no lights or any other electrical load, then the dynamo forces the
> remaining 6 amps through the ammeter and into your battery.
> This is bad if you drive 1000km at 100kmph because your battery gets
> overcharged, hence overheated, and may well get bent plates inside as
> well as boiling off of much of the acid.
> It is also bad if you set the third brush to produce about 4 amps AND
> you have your side & headlights on, thus taking 8 amps in addition to
> the ignition coil's 2 amps. You will then be discharging the battery at
> about 6 amps - let's hope you don't get caught in heavy traffic!
>
> I haven't tried this, but I imagine that if you install a voltmeter in
> the car, and switch resistance in and out of the field coil circuit so
> as to keep the voltmeter indicating as near 13.5 volts as possible, you
> would have most of the advantages of a CVC dynamo, (but with one serious
> disadvantage if you are forgetful like me).
>
> Hope this helps.
>
> Cheers ...... Pat Mullen (1936 MG 2-seater NA0895)
>
>
> hiro@octagongarage.com wrote:
> >
> > Hi,
> >
> > We had largest MG event in Japan on Saturday 26th, and I
> > participated with my J2. The return trip was just 1000km
> > (625miles), my J2 happyly cruised around 60mph - just one thing
> > I noticed, maybe oil pump relief valve was sticking since
> > pressure when cold become over 160LB!
> >
> > On the way to the site the third brush of my J2 dymano was
> > broken up in pieces (as you know it is very thin), then I had
> > to purchase a spare battery.
> >
> > Does someone explain me what is the mechanism of this type of
> > dynamo?
> >
> > Hiro
> > Kobe-Japan
> >
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